As modern electronic circuit boards evolve toward increased circuit and component densities, thorough cleaning of the board after soldering becomes more important. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with flux and thereafter passing the flux-coated board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes solder fusion. Commonly used solder fluxes generally consist of rosin, either used alone or with activating additives, such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the remaining flux and flux-residues are often removed from the circuit boards with an organic solvent. The requirements for such solvents are very stringent. Defluxing solvents should have the following characteristics: be low boiling, be nonflammable, have low toxicity and have high solvency power, so that flux and flux-residues can be removed without damaging the substrate being cleaned.
While boiling point, flammability and solvent power characteristics can often be adjusted by mixing different solvents together, the mixtures that are formed are often unsatisfactory because they fractionate to an undesirable degree during use. Such solvent mixtures also fractionate during distillation, which makes it virtually impossible to recover and reuse a solvent mixture with the original composition.
On the other hand, azeotropic mixtures, with their constant boiling point and constant composition characteristics, have been found to be very useful for these applications. Azeotropic mixtures exhibit either a maximum or minimum boiling point and they do not fractionate on boiling. These characteristics are also important when using solvent compositions to remove solder fluxes and their residues from printed circuit boards. Preferential evaporation of the more volatile components of the solvent mixture would occur if the mixtures were not azeotropes or azeotrope-like and could result in mixtures with changed compositions having possibly less-desirable solvency properties, such as lower rosin flux solvency and lower inertness toward the electrical components being cleaned. The azeotropic character is also desirable in vapor degreasing operations where redistilled solvent is generally employed for final rinse cleaning.
Thus, vapor defluxing and degreasing systems act as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distributions will result which could detrimentally affect the safety and efficacy of the cleaning operation.
A number of chlorofluorocarbon-based azeotropic compositions have been discovered and, in some cases, used as solvents for solder flux and flux-residue removal from printed circuit boards and also for miscellaneous degreasing applications. For example: U.S. Pat. No. 3,903,009 discloses the ternary azeotrope of 1,1,2-trichlorotrifluoroethane with ethanol and nitromethane; U.S. Pat. No. 2,999,815 discloses the binary azeotrope of 1,1,2-trichlorotrifluoroethane and acetone; U.S. Pat. No. 4,767,561 discloses the ternary azeotrope of 1,1,2-trichlorotrifluoroethane, trans-1,2-dichloroethylene and methanol.
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes. This fact obviously complicates the search for new azeotropic compositions which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropes or azeotrope-like compositions which have improved solvency characteristics and particularly greater versatility in solvency power.